During operation, telecommunication equipment requires timing synchronization for proper communication purposes. Traditional synchronization methods utilize expensive specialized circuits to provide synchronization signals. Currently, with the prevalence of packet-based networks, such as, for example, Ethernet, cost can be reduced for timing synchronization by transmitting timing synchronization signals between telecommunication equipment within the packet-based network. However, due to the store-and-forward operation of packet-based networks, the packets carrying the synchronization information will experience an uncertain delay, which will affect the accuracy of the synchronization. This uncertainty in delay is commonly referred to as packet delay variation (PDV). To improve timing synchronization accuracy, it is necessary for such delay to be significantly reduced or eliminated.
A circuit emulation service (CES) is defined as the transport of a time-division multiplexing (TDM) based service over a packet-based network. The key for successful CES transport is the preservation of the TDM-based service clock between interworking functions (IWFs). CES timing recovery methodologies fall into two categories: differential and adaptive. The differential timing relies on the presence of an external high quality timing reference at each IWF to measure and reconstruct the service clock. The adaptive timing relies on reconstructing the service clock through statistical methods based on packet arrival at the egress IWF.
Timing emulation service (TES) also transports timing information over a packet-based network. However, this service is not associated with the transport of a TDM-based service. TES is implemented by a unique IWF, different from the CES IWF, which transports timing information suitable for several different applications:
Syntonization—Frequency information is transported. The maximum time interval error (MTIE) of the recovered timing service will be unbounded but controlled.
Synchronization—Both frequency and phase information are transported. MTIE of the recovered timing service will be bounded and controlled.
Time-of-day—Timing information will be sent between IWFs related to a time-of-day source, such as, for example, a time stamp, that represents a time value relative to an epoch and a counting rate.
The performance of adaptive time transfer protocols is influenced by packet delay and delay variation. Such protocols send timing information as a series of time-bearing packets over a packet switched facility. If these time-bearing packets are sent in one direction, from a timing server to a timing client, the overall delay that these packets experience in the network results in a timing offset at the timing client. If the delay is constant, then there will be a constant time error between the timing server and the timing client. However, if the delay varies, then this delay variation will also be present in the timing information recovered by the timing client.
Adaptive time transfer protocols that are capable of transferring time-bearing packets in two directions of communication (two-way mode) are capable of measuring the round-trip delay between the timing client and the timing server. If the round-trip delay is then divided by two, an estimate of the one-way delay may then be made. Examples of adaptive time transfer protocols that operate in this two-way mode are IEEE 1588 version 1 (V1) and version 2 (V2), as well as network time protocol (NTP).
However, the ability of these two-way protocols to accurately provide correct timing information at the timing client is dependent on two basic factors. The first factor is the symmetry of the delay characteristics from the timing server to the timing client and from the timing client to the timing server. The second factor is the rate of change of these delay characteristics relative to the exchange of messages between the timing server and timing client.
Delay symmetry between the timing server and timing client is typically influenced by the residence time that these time-bearing packets spend in intermediate switches in the network. Although there are techniques to measure the residence time and make on-the-fly corrections, see, for example, IEEE 1588 V2, such corrections must be done at each intermediate node that passes these time-bearing packets. Delay asymmetries may be caused by a number of factors including intermediate switch loading, asymmetric packet traffic patterns, or even different packet routes.
The characteristics of the delay asymmetry are also very important. If an asymmetry is fixed, it will result in stable timing offset at the timing client. In this case, MTIE and recovered frequency will be stable. If the timing client is providing its recovered timing information for telecom applications, DS1/E1 slip buffers will not overflow or underflow under stable asymmetric conditions in packet-based networks.
Variation in delay asymmetry, however, may cause the MTIE of the recovered timing at the timing client to change. In the case of the one-way adaptive timing protocol, PDVs will be directly transferred to timing changes at the timing client. If rate of change of the delay variation is particularly slow, such as variations of hours, days, or longer, the recovered MTIE will tend to track the PDV.
Two-way timing protocols that are able to measure the round trip delay may not be able to accurately measure the one-way delay or delay variation. Such measurement errors will also result in the production of wander at the client IWF due to these PDV events. Therefore, the relationship between network PDV and the resulting timing performance (wander generation) of adaptive protocols needs to be better understood.
Since network PDV can adversely affect the timing performance of adaptive time-transfer protocols, such as, for example, IEEE 1588 V1, V2, NTP, etc., a series of tests that directly correlate timing performance at the timing client to PDV are also needed.
Timing performance tests mentioned in the appendix of ITU-G.8261 standard are based on packet loading in a series of Ethernet switches. Though this type of loading will cause PDV, it is not deterministic in nature. Dependencies on the switch design, actual data traffic and other factors will cause the actual PDV of packet traffic to possess these indeterminate properties.
Accordingly, efficient and controlled approaches are needed for the testing and evaluation of adaptive timing characteristics of packet-based timing protocols. More specifically, a need exists for the evaluation of the timing recovery performance of CES and TES systems.